Abstract: A backup mandrel apparatus to engage the inner surface of a metal tube being welded to support the weld bead and to keep a molten weld puddle from sagging and falling through the tube wall during welding. The apparatus includes a flexible inflatable bladder surrounded by a split expandable mandrel, an expandable band typically surrounding a central portion of the split expandable mandrel, insulation pads surrounding the mandrel adjacent to the expandable band and insulation material covering the inflatable bladder. The insulation pads preferably have a compressed thickness slightly greater than the band thickness. This assembly is inserted within the tubular workpiece to be welded. In use, the inflatable bladder is inflated with low air or water pressure to expand the mandrel to uniformly expand the expandable band to press the insulation pads against the inner side of the tubing workpiece being welded while leaving either no gap or a narrow gap between the band and the inner tube wall surface.